Mitochondrial content and capillarization ultimately have to reflect on performance which is encapsulated in your signature. If they didn’t, you wouldn’t see an increase in performance. This research is yet another example of how Xert’s methods are supported by science.
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Thanks for sharing! To me, there were 2 key points that stood out:
From their “Key Points” summary:
Greater training volumes and higher exercise intensities are associated with larger increases in mitochondrial content. Thus, training load (intensity × volume) is the most suitable predictor, underlining that higher exercise intensities can compensate for lower training volumes and vice versa.
This resonates with my own experience. Looking back over the past ~10 years, the periods where my training load was consistently the highest (typically around 4 stars, or roughly ~110 Training Load) also corresponded to my best fitness, both in terms of signature & in race results/segment efforts.
Those periods weren’t just characterized by higher intensity, they also coincided with my highest weekly training volume, usually 12–15 hours/week versus ~8 hours/week when my fitness is lower.
I completely appreciate that many athletes are time-constrained. But for me, the biggest gains have always come when I was able to consistently increase my total volume. Interestingly, that also aligns with XFAI plans, which often forecasts the greatest improvements when the max weekly training hours are increased.
Another interesting point from the abstract:
Per total hour of exercise, SIT was~2.3 times more efficient in increasing mitochondrial
content than HIT and~3.9 times more efficient than ET, while HIT was ~1.7 times more efficient than ET.
With all the discussion around Zone 2 over the past few years, I think it’s a useful reminder that high-intensity training is an incredibly time-efficient way to stimulate mitochondrial adaptations!
Of course, we also know you can’t do high-intensity work every day. It comes with greater fatigue and requires more recovery. That’s why I’ve always liked the phrase “earn your Zone 2.” Build your fitness with targeted high-intensity work (appropriate to your unique fitness level), then use Zone 2 to accumulate additional training load without exceeding your ability to recover.
All put together, it makes sense that the best way to improve your fitness is a sustainable combination of high- and low-intensity. Xert is quite good at that 
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I subscribe to idea that you do as much volume that you can recover from properly at an intensity which doesn’t compromise your ability to do the higher intensity sessions properly.
It’s a lot easier to type that than do it though
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I don’t think this intervention was controlled by load, which renders the whole exercise pretty much worthless. Please correct me if I’m wrong (on the control).